In optical fiber communication systems, and optical fiber light transmission systems generally, it is typically important to increase the efficiency with which light from a source is coupled into the fiber. One of the most common applications is in communications systems to couple light from a single-transverse-mode light source, e.g., semiconductor laser, into a single-mode optical fiber. Historically, there has been a trade-off between high coupling efficiency and ease of manufacture. The following examples illustrate this principle:
1. Bulk optics PA1 2. Hyperbolic fiber endfaces PA1 3. Single-angle wedge-shaped fiber endfaces PA1 4. Double-angled wedge-shaped fiber tips
This is a popular technique in which lens are used to focus the light from the semiconductor laser onto the endface facet of the fiber. It can provide high-coupling efficiency if adequate lenses are used. The fact that multiple components, typically 1 or 2 lenses, are needed greatly increases the complexity of implementation and the reliability risk, however.
The technique has provided coupling of 99% of the power from a single-mode laser into a single-mode fiber by using a high-power infrared laser to machine a hyperbolic surface on the fiber endface or tip. A hyperbola is the ideal shape for fiber coupling. Unfortunately, it is extremely difficult to accurately manufacture these fiber tips with sufficient yield that would warrant implementation in large-scale manufacturing of fiber-coupled single-mode laser modules.
This has been used to couple 980 nanometer pump lasers to single-mode fibers. While it has the positive feature of being easy to manufacture, the achievable coupling efficiency has been limited to between 65 and 70%.
This technique is a compromise between hyperbolic fiber endfaces and the single-angle wedge-shaped fiber endfaces described above. It provides higher coupling efficiency than single-angle wedge-shaped endfaces, but not as high as hyperbolic endfaces. It is significantly easier to manufacture and implement than hyperbolic endfaces, but not as easy as single-angle wedge-shaped fiber endfaces. To completely specify and manufacture the double-angled wedged-shaped fiber tip, four independent angles, having one of only two different values, and the location of three lines of intersection must be fabricated with sufficient precision.